The present invention generally relates to a printing method, and more particularly, to a thermal transfer printing method for effecting printing on a paper sheet or the like by a printing means such as a thermal head, etc. through employment of an ink material containing at least a coloring material, and a printing apparatus employed for executing the method.
Recently, with respect to the thermal transfer printing method and printing apparatus for effecting such a method as referred to above, there has been proposed an arrangement in which, for example, through employment of an ink sheet prepared by forming an ink material layer of about 3.mu. thick with a hot-melt binder material and a pigment coloring material, on a surface of a heat-resistant base material such as a polyethylene terephthalate (referred to as PET hereinafter) film and condenser paper or the like, the ink material is adapted to be directly fused and transferred onto a recording medium by a receiving head, to thereby obtain a recorded item.
Where the coloring material is composed of a subliming dye which is to be transferred for printing by sublimation or heat diffusion, there have also been known a thermal transfer printing method and a printing apparatus therefor in which, similarly, with use of an ink sheet prepared by forming an ink material layer containing a subliming dye and a binder material, on the surface of the heat-resistant base material such as PET film, condenser paper or the like, the subliming dye as the coloring material is directly transferred onto a recording medium having a dyeing property or dye affinity by a printing head so as to obtain a recorded item.
One example of the conventional thermal transfer printing methods and printing apparatuses as referred to above will be explained below with reference to FIG. 4.
In FIG. 4, the known thermal transfer printing apparatus generally includes an ink sheet 6, a thermal head 8, and a printing signal source 21 connected to the thermal head 8. The ink sheet 6, made by disposing an ink material layer 5 composed of a coloring material and a binder material on a heat-resistant base member 4, and an image receiver 11, such as recording paper or the like, are held under pressure between the thermal head 8 and a platen 7 while the thermal head 8 is heated according to the signal from the printing signal source 21 to thereby selectively raise the temperature of the ink material. The image receiver 11 is transported in a direction of an arrow 52 through a set of transport rollers 51, while the ink sheet 6 is transported in a direction of an arrow 53 by an ink sheet winding roller 10. When the ink sheet 6 is separated from the image receiver 11, part of the coloring material of the ink material layer 5 is transferred onto the image receiver 11 to thereby provide a printed image 54 on the image receiver.
The conventional thermal transfer printing apparatus as described above is arranged to effect the thermal transfer printing onto the image receiver through a temperature rise of the ink material layer.
In the case of subliming transfer, in which the coloring material composed of the subliming dye is transferred by sublimation or heat diffusion, the transfer printing becomes possible only when the surface material of the image receiver is of a material with a dye property or affinity for dye, and therefor the transfer printing sensitivity markedly differs according to the surface materials of the image receiver. For example, if general paper sheets are employed for the image receiver, the printed image by the transfer of dye is hardly obtainable. Moreover, the printing sensitivity also varies depending on the contact state between the ink sheet and the image receiver. For example, in an image receiver having a surface with concave and convex portions or undulations, poor contact between the ink material layer and the image receiver takes place, thus making it impossible to provide a uniform printed image.
On the other hand, in the case of a melting transfer in which the ink material layer is melted for a low viscosity by the temperature rise, the printing tends to be affected by the state of contact between the ink sheet and the image receiver. For example, when a plain paper sheet used for copying apparatus or the like is utilized for the image receiver, the surface thereof has undulations of at least 25 .mu.m or thereabout, due to the presence of the fibers of the paper, whereas the thickness of the ink material layer of the general ink sheet is several .mu.m. Accordingly, when such a plain paper sheet is used for an image receiver, contact between the ink material layer and the image receiver becomes non-uniform, and thus favorable printing images cannot be obtained. Furthermore, since the ink material recorded by the melting transfer has little adhesion with respect to the paper, the strength of the printed image with respect to friction on the surface of the image receiver (i.e. the fixing characteristic of the image) is very poor.